1. Field of the Invention
The present invention relates to voltage controlled oscillators or VCOs.
The present invention relates more particularly to an RF circuit comprising a voltage controlled oscillator delivering an RF signal, a phase locked loop to control the voltage controlled oscillator, a modulation circuit receiving the RF signal and delivering a modulated signal comprising at least one RF harmonic component capable of disturbing the voltage controlled oscillator by injection pulling.
2. Description of the Related Art
In radio frequency circuits using VCOs, the performances of the VCOs are deteriorated by leaks of harmonic signals, due to “injection pulling” also known as “injection locking”.
The present invention aims to remove, or at least to reduce, the injection pulling in the VCOs.
For a better understanding, FIG. 1 shows a classical application of a VCO in the area of radiotelephony. In this Figure a circuit RFCT comprising a VCO, a circuit forming a phase locked loop or PLL circuit and a modulation circuit TXCT can be distinguished.
The VCO delivers to the modulation circuit TXCT a voltage V1 the frequency F1 of which is controlled by the PLL circuit. For that purpose, the PLL circuit comprises a divide-by-N frequency divider DIVN that receives the voltage V1 at input and that delivers a signal of frequency F1/N to one input of a phase comparator PCOMP. The phase comparator receives a reference frequency FREF at another input. This frequency FREF is, for example, delivered by a divide-by-M divider DIVM the input of which is linked to a quartz oscillator. The output of the comparator delivers a control signal Vcont that is applied to one control input of the VCO through a loop filter LOOPF having a determined bandwidth. The signal V1 is therefore frequency and phase controlled and its frequency F1 is equal to N/M*FREF.
Here the circuit TXCT is a data transmission circuit by phase modulation IQ (quadrature modulation PM) provided for a mobile telephone for example.
The circuit TXCT receives an analog signal Sx and the signal V1 from the VCO at input, and delivers a signal RFSx intended to be applied to an RF antenna, that is phase modulated by means of two quadrature signals I and Q.
The circuit TXCT comprises a divide-by-K divider DIVK the input of which receives the signal V1 and the output of which delivers a modulation PM carrier FRF, FRF being equal to F1/K, K generally being equal to 2 or to 4. The signal Sx is digitised by a converter ADC, then it is applied to an encoder modem CODEM then is applied again to a processor IQGEN. The processor IQGEN delivers, in a baseband of frequency FBB, phase I and quadrature Q signals. The signal I is applied to one input of a mixer IMIX through an amplifier IAMP, and the signal Q applied to one input of a mixer QMIX through an amplifier QAMP. The mixer IMIX receives the carrier FRF at another input and the mixer QMIX receives the carrier FRF phase-shifted by 90° at another input, delivered by a phase shifter DPH. The outputs of the mixers IMIX, QMIX are applied to an adder IQAD that delivers the modulated signal RFSx. The signal RFSx is applied to an output amplifier RFAMP the output of which forms the output of the transmission circuit TXCT.
The signal Sx generally contains data to be transmitted, such as a coded voice for example, and has a spectrum of frequencies representative of the modulation schema provided for by the standard implemented (such as GMSK in GSM for example). Considering, as an example, that the signal Sx is a single tone, the circuit IQGEN then delivers two pure quadrature sine curves I=cos (FBB) and Q=sin (FBB). The result of the phase modulation IQ is, in this case, a single tone of frequency FRF+FBB the image component FRF−FBB of which is removed by the quadrature modulation, and the carrier FRF of which is also removed.
Due to imperfections in the modulation circuit, or “non-linearity”, the output signal comprises in addition to the wanted component H1 of frequency FRF+FBB, harmonics H2, H3, H4, . . . . At least one of these components is proximate to the oscillation frequency F1 of the VCO. It is the first harmonic H1 (wanted component) when the divider DIVK does not exist or has a division value equal to 1 (K=1), the second harmonic H2 when the divider DIVK is a divide-by-2 divider (K=2) or the fourth harmonic H4 when the divider DIVK is a divide-by-4 divider (K=4). When K=2, the frequency of the second harmonic H2 is in fact equal to 2FRF+2FBB (i.e., F1+2FBB) and is very proximate to the center frequency F1 of the VCO as the frequency of the baseband FBB is low before the carrier FRF, generally in the order of a few Gigahertz. Similarly, when K=4, the fourth harmonic H4 has a frequency of 4FRF+4FBB (i.e., F1+4FBB) that is proximate to the center frequency of the VCO.
It is well known that the involuntary injection of this harmonic component into the core of the VCO, by various spurious paths, deteriorates the performances of the VCO.
Various methods are known to overcome this disadvantage.
One known method involves producing the VCO on a substrate distinct from the one bearing the phase modulation IQ circuit TXCT. This substrate is arranged in a sheathed case and comprises means for connecting to the circuit TXCT that are equipped with insulating barriers preventing the spurious harmonics sent by the circuit TXCT from “rising” to the core of the VCO. These barriers generally comprise filters, “balun” type connectors, insulators, buffer circuits . . . and must be provided in all the conduction paths linking the VCO to the circuit TXCT, including the power supply paths. This solution is however complex to implement and increases the cost price of the RF circuits, which is passed on at the end of the chain to the selling price of the mobile telephones.
Other methods are based on providing a phase modulation IQ circuit architecture in which the VCO is quite insensitive to the spurious harmonics.
Therefore, the heterodyne systems use several VCOs and several cascade-arranged mixers, and a premodulation stage using an intermediate frequency IF. In the output stage, the frequency of the modulated signal is clearly offset in relation to the natural frequency of the VCO, and the harmonics capable of interfering with the VCO are harmonics and/or mixing products of high rank that are greatly attenuated.
However, the disadvantage of the heterodyne systems is that they require the use of at least two VCOs, as well as additional mixers and filters, and are therefore costly and bulky.
Another solution to counter the injection pulling includes providing a copy loop in the VCO. This copy loop allows harmonic frequencies to be obtained that are offset in relation to the center frequency of the VCO, and are located outside its bandwidth (determined by the loop filter). However, this solution also requires using several VCOs, generally three VCOs at least.
Various architectures of RF modulation circuits or of VCOs that are quite insensitive to injection pulling are described particularly in U.S. Pat. Nos. 6,321,1074, 5,144,260 and 6,281,758.